Further Covalent bonding
Learning Objectives:
• Use dot and cross diagrams to represent covalent bonding
• Understand when to apply the expansion of the octet rule.
Key Words:
Covalent, sharing, bonding, octet rule.
How many covalent bonds?
When covalent bonds form the unpaired electrons pair up so
that the bonded atoms obtain a noble gas configuration,
generally having 8 electrons in the outer shell.
This is called the Octet Rule.
However this is not always possible to achieve;
• There may not be enough electrons to react an octet.
• More than four electrons may pair up when bonding occurs
(expansion of the octet)
Not enough electrons to reach
the octet.
Within period 2 beryllium and boron both form compounds
with covalent bonding.
However, Be and B don’t have enough unpaired electrons to
reach an octet. So they pair up all the electrons they have
available.
Example: Boron Tri fluoride, BF3
•
•
•
•
•
•
Boron has 3 electrons in its outer shell
Fluorine has 7.
3 covalent bonds are formed
Each of Boron’s 3 electrons are paired
6 electrons surround B
Each of the 3 Fluorine atoms have 8
electrons – are stable
Expansion of the octet.
Elements in groups 5-7 in period 3 also behave differently
As we move down the periodic table more of the outer
electrons can take part in bonding. So some molecules have
more than 8 electrons in the outer shell.
Expansion of the octet
Example: Sulphur hexafluoride, SF6
•
•
•
•
•
•
Sulphur has 6 electrons in its outer shell
Fluorine has 7.
6 covalent bonds are formed
Each of sulphur’s 6 electrons are paired
12 electrons surround S
Each of the 6 Fluorine atoms have 8
electrons – are stable
Expansion of the octet.
Elements that expand their octet:
Group 5
P
As
Group 6
Group 7
Se
Br
Te
I
Expansion
of the octet
S
Cl
At
General Rules
• Non-metal atoms in Group 5 can form 3 or 5 covalent bonds
depending on how many electrons are needed for bonding.
• Non-metal atoms in Group 6 can form 2,4 or 6 covalent
bonds depending on how many electrons are needed for
bonding.
• Non-metal atoms in Group 7 can form 1,3,5 or 7 covalent
bonds depending on how many electrons are needed for
bonding.
A better rule.
• Unpaired electrons always pair up
• The maximum number of electrons that can pair up is
equivalent to the number of electrons in the outer shell.
Questions.
1. Draw dot and cross diagrams for:
a) PCl4+
b) H3O+
c) H2F+
2. Draw dot and cross diagrams for:
a) BF3
b) PF5
c) SO2
d) SO3
Shapes of Molecules and Ions
Learning Objectives:
• Explain how the shape of a simple molecule is determined.
• State that lone pairs of electrons repel more strongly than
bonded pairs.
• Explain and predict the shapes of, and bond angles in,
molecules and ions.
Key Words:
Electron pairs, bonding pairs, lone pairs, double bonds
Shapes of Molecules and Ions
Electron Pair Repulsion Theory.
The shape of a molecule is determined by the number of
electron pairs surrounding the central atom.
• As all electrons are negatively charged, each electron pair
repels the others near it.
• The electron pairs arrange themselves as far apart as
possible.
Shapes of Molecules and Ions
Drawing 3D Diagrams
Molecules with Lone Pairs
A lone pair (l.p) of electrons is more repulsive than a bonding
pair. IT is more electron dense.
Therefore a lone pair repels more than a bonded pair.
Lone pair/Lone pair > bonded pair/lone pair > bonded pair/bond pair
Molecules with Lone Pairs
Lone pair/Lone pair > bonded pair/lone pair > bonded pair/Lone pair
Molecules with Double Bonds
A double bond counts as 2 bonded pairs. To work out the shape
for a molecule with double bonds each double bond is treated
as a bonded region in the same way as a bonded pair
Shape of CO2
Shapes of Ions
The principles discussed previously still apply to molecular
ions.
e.g. the shape of the ammonium ion, NH4+
• There are 4 electron pairs
around the central atom
• The shape will be tetrahedral
Questions.
1. Draw dot and cross diagrams and predict the shapes and
bond angles for:
a) H2S
b) AlCl3
c) SiF4
d) PH3
2. Draw dot and cross diagrams and predict the shapes and
bond angles for :
a) NH4+
b) H3O+
c) NH23. Each of the following has at least one multiple bond. Draw
diagrams and predict the shapes and angles.
a) HCN
b) C2H4
c) SO2
d) SO3